Nowadays, with increasing awareness of environmental protection, more and more electronic devices are designed in views of power-saving concepts. Therefore, it is very important to increase the operating efficiency of the electronic devices. In recent years, for example, light emitting diodes (LEDs) capable of emitting light with high luminance and high illuminating efficiency have been developed. In comparison with a common incandescent light, a LED has lower power consumption, long service life, and quick response speed. Until now, LEDs are widely used in many aspects such as household lighting devices, automobile lighting devices, handheld lighting devices, indicator board displays, and the like.
FIG. 1 is a schematic circuit diagram illustrating a conventional lamp driving device. As shown in FIG. 1, some lamps 1A, 1B and 1C are installed at different positions within a house according to the practical requirements. In addition, a lamp driving device 1 is used for simultaneously driving these lamps 1A, 1B and 1C. The lamp driving device 1 is a two-stage power conversion circuit. In particular, the two-stage power conversion circuit comprises a first-stage circuit 11 and a second-stage circuit 12. The first-stage circuit 11 is an AC-to-DC converter. The first-stage circuit 11 is configured to convert an input voltage Vin into a bus voltage Vbus having a constant voltage value, and transmit the bus voltage Vbus to second-stage circuit 12. The second-stage circuit 12 comprises a first DC-to-DC converter 121, a second DC-to-DC converter 122 and a third DC-to-DC converter 123. The output terminals of the first DC-to-DC converter 121 are connected with a first lamp connector 131A and 131B. The output terminals of the second DC-to-DC converter 122 are connected with a second lamp connector (132A, 132B). The output terminals of the third DC-to-DC converter 123 are connected with a third lamp connector (133A, 133B). The first lamp connector (131A, 131B), the second lamp connector (132A, 132B) and the third lamp connector (133A, 133B) are respectively connected with the lamps 1A, 1B and 1C. Through the three lamp connectors, three output voltages Vo1, Vo2 and Vo3 are respectively transmitted to the lamps 1A, 1B and 1C.
In a case that the lamp switch 10 is in the on status, the input voltage Vin is transmitted to the input terminal of the first-stage circuit 11 through the lamp switch 10. By the first-stage circuit 11, the input voltage Vin is converted into a bus voltage Vbus having a constant voltage value (e.g. about 52V). The bus voltage Vbus is inputted into the input terminals of the DC-to-DC converters 121, 122 and 123. By the DC-to-DC converters 121, 122 and 123, the voltage of the bus voltage Vbus is reduced. As a consequence, the first output voltage Vo1, the second output voltage Vo2 and the third output voltage Vo3 are outputted from the DC-to-DC converters 121, 122 and 123, respectively.
Moreover, the lamp driving device 1 is configured to drive plural LED lamps that comply with the same specification. As known, the brightness value of the LED is in a direct proportion to the current. For achieving identical brightness values of these LED lamps, the DC-to-DC converters 121, 122 and 123 should be manufactured according to the same specification. Consequently, each of the magnitudes of the output voltages Vo1, Vo2 and Vo3 is, for example, about 50V, and the magnitudes of the output currents Io1, Io2 and Io3 are substantially identical. However, due to different fabricating conditions of the DC-to-DC converters 121, 122 and 123 are usually different and the component tolerance, the performance of the DC-to-DC converters 121, 122 and 123 may be different. In other words, the magnitudes of the output currents Io1, Io2 and Io3 provided by the DC-to-DC converters 121, 122 and 123 are different in most situations.
Moreover, since both of the first-stage circuit 11 and the second-stage circuit 12 consume power, there is a power loss after the input voltage is converted by the first-stage circuit 11 and the second-stage circuit 12 into the output voltages required for powering the lamps. Under this circumstance, the power efficiency is reduced, the power consumption is increased, and the operating efficiency of the lamp driving device is unsatisfied. Moreover, since each of the DC-to-DC converters 121, 122 and 123 has an inner controlling circuit, the circuitry is complicated, and it is difficult and costly to integrate these DC-to-DC converters 121, 122 and 123 into a single module. In other words, if the number of lamps to be driven is increased (e.g. if the lamp driving device is used to drive six lamps), the circuitry of the lamp driving device should be re-designed without directly replacing the module. As known, the process of re-designing and fabricating the lamp driving device is time-consuming and costly.
Therefore, there is a need of providing an improved lamp driving device so as to obviate one of the above-mentioned drawbacks encountered from the prior art.